Due to the increasing energy demands, offshore oil and gas production is moving into deeper waters. For providing an efficient and secure production, processing facilities are being installed at the ocean floor. Such subsea installations may include a range of components, including pumps, compressors, and the like as well as a power grid for operating the components. The power grid may, for example, include a subsea transformer, subsea switchgear, and subsea variable speed drives. The components of the subsea installation are to be protected from the surrounding sea water, in which pressures of 300 bar or more may prevail (at installation depths of 3,000 m or more).
Electrical power may be provided to the subsea installation from a top side installation, such as a fixed or floating offshore platform or vessel, or an onshore side. The electric connection to the top side power source may be provided by an electric power transmission cable that may be included in an umbilical or may be provided as a subsea power cable. This way, AC electric power may, for example, be transmitted over several kilometers, and in some installations over even more than 100 kilometers.
When such electric power transmission cable is connected to a voltage source, the impedance of the cable may cause reflections and over-voltages within the cable or in equipment connected to the far end of the cable (e.g., in subsea devices).
In some installations, a transformer may be connected to the far end of the cable, which may result in increased over-voltages or reflections, and which may cause an even bigger challenge in designing such power transmission system.
To prevent that such over-voltages and reflections cause any problems, it is known to design the components of such power system with sufficient over-voltage capability. Accordingly, the power transmission cable and the subsea transformer and other components may be configured to handle such reflections and over-voltages. However, designing the equipment for higher voltages generally leads to an increased size and weight of the components, and also, such components will be considerably more expensive. The costs for transportation of such subsea components on a vessel increase with required space and weight are generally very high, so that it is desirable to keep these subsea components as small and lightweight as possible. It is also desirable to keep the power system simple, since an increase in complexity would lead to additional costs and would also render the system more prone to failures.